1. Field
The invention is in the field of apparatuses and methods for modifying suspension systems used on motor vehicles, and in particular directed to those which are used to modify double wishbone suspension systems, wherein the angular relationships of the suspension system control arms are modified so as to change vehicle handling characteristics.
2. State of the Art
The handling characteristics of motor vehicles, such as automobiles, are largely determined by the vehicle's suspension system, particularly the front suspension system. The purpose of the suspension system is to provide the vehicle driver and passengers with a smooth ride while at the same time providing the driver with adequate control over the vehicle, particularly when accelerating, braking, cornering, and when riding over an uneven road surface. The design of the suspension systems, as received from the factory, are therefore necessarily a compromise between performance, handling, smooth ride, and safety, with the emphasis on safety.
A double wishbone suspension system is a type of suspension system commonly used on automobiles. A key parameter in the design of such a suspension system, particularly with regard to safety and handling, is the camber curve. On double wishbone suspension systems, the upper and lower control arms form a four-bar linkage with the vehicle frame and with the respective spindles upon which each wheel and tire rotate. A coil spring spanning between each lower control arm and the vehicle frame transfers the vehicle weight to respective wheels and tires while allowing the respective control arm ends, spindles, wheels, and tires to move up and down in response to vehicle acceleration, braking, turning, and road conditions, in an arcuate path. This path, as viewed from the front or the rear of the vehicle, is the camber curve. The camber curve is defined by the specific geometry of the vehicle frame, control arms, and spindle, which comprise the four-bar linkage. At any given vertical position of the spindle, wheel, and tire relative to the vehicle frame, the center of the camber arc is located at a specific instantaneous center point, or I.C.P., which point moves as the suspension system moves through the camber curve.
The typical double wishbone motor vehicle suspension system as designed and built by the manufacturer has camber curves which curve away from the vehicle when the suspension is moved up or down from the static position, and, therefore, has instantaneous center points which are on the side away from the vehicle. Thus, when the vehicle turns, the tire tread does not remain perpendicular to the road, but rather the tires ride on their edges due to the rolling action of the vehicle caused by weight shifting away from the direction of turning. Both tires are loaded on the shoulder facing away from the direction of the turn, with the outside tire being particularly heavily loaded. The shoulders facing the direction of the turn are lightly loaded, or not loaded at all, and in some cases the entire inside tire can lift off the surface of the road. The sidewall of the outside tire may even touch the highway surface in a severe turn. The result is excessive tire wear, particularly to the front tires, on the outside tire tread and shoulder, along with poor lateral cornering traction and poor vehicle cornering performance. Also, even when the vehicle is not turning, when a tire hits a bump and moves upward, the tire rides on its outer edge, and when the tire hits a dip and moves downward, the tire rides on its inside edge.
The reason automobile manufacturers design suspension systems such as the dual wishbone suspension systems of their rear wheel drive automobiles with outwardly curving camber curves is to build understeer into the vehicles to prevent oversteer. Understeer occurs in a hard turn, when the front tires lose lateral traction before the rear tires do, causing the vehicle to turn less or track in a straighter path than the actual steering position of the wheels. Oversteer is the opposite, wherein the rear tires slide laterally first, causing the vehicle to turn faster than the steering position of the front wheels. Understeer has been designed into most automobiles to prevent the occurrence of trailing throttle oversteer. Trailing throttle oversteer occurs when the vehicle driver reduces the throttle position in a turn, transferring weight to the front tires by adding a braking component to the rear tires from compression braking from the engine. This can induce oversteer by reducing traction of the rear tires.
Some designs in use today incorporate such an extreme understeering suspension design that the lack of turning ability may be as unsafe as oversteer. Some vehicle owners have attempted to revise the design of the suspension system to a more neutral geometry so as to reduce wear of the front tires and to balance the handling characteristics to a more desirable balance between understeer and oversteer. This has typically been attempted by reworking the vehicle frame to move the pivot points of the control arms on the frame, or by manufacturing special spindles having greater height to further space the pivot points of the control arms adjacent the wheels. While these changes alter the camber curves in a favorable manner, such changes require either extensive modification of the frame (not possible in some cases) or manufacture of costly new taller spindles. Neither of these may be economical nor within the means of most owners of motor vehicles in need of modification.